Photovoltaic solar cells are well known in the art. Such cells typically comprise a P-N or N-P semiconductor junction, and metal electrodes formed on the cell's top and bottom surfaces. The electrodes on the top (or front) sides of the solar cells are typically formed as a group of fingers connected by one or more buses, while the electrodes on the bottom (or back) sides of the solar cells are typically formed as uninterrupted layers (see, for example, U.S. Pat. Nos. 4,434,318 and 4,443,652).
As is well known in the art, when radiation of an appropriate wavelength strikes the top (or front) side of the solar cell, the radiation will generate electron-hole pairs in the cell and thereby produce a potential difference at the semiconductor junction and thus across the electrodes. The electron-hole pairs, in effect, move across the junction in opposite directions so as to provide an electric current which is capable of driving an external circuit.
By appropriately interconnecting the electrodes of two or more solar cells, an array of cells can be provided which will meet certain power requirements. For example, when the electrodes of two or more solar cells are connected in parallel (i.e., where the top electrode of a first cell is connected to the top electrode of an adjacent second cell, and the bottom electrode of the first cell is connected to the bottom electrode of the adjacent second cell), the current provided by the entire array will be the sum of the currents provided by each of the individual cells. Similarly, when the electrodes of two or more solar cells are connected in series (i.e., where the top electrode of a first cell is connected to the bottom electrode of an adjacent second cell, and the bottom electrode of the first cell is connected to the top electrode of an adjacent third cell, or to a load), the voltage provided by the entire array will be the sum of the voltages provided by each of the individual cells.
For convenience of description, the term "parallel array" will sometimes hereinafter be used to refer to an array of solar cells of the type wherein the cells are connected in parallel, and the term "series array" will sometimes hereinafter be used to refer to an array of solar cells of the type wherein the cells are connected in series.
In practice, it has proven to be a relatively simple matter to electrically interconnect a plurality of solar cells in parallel so as to produce a parallel array of cells. More specifically, the desired electrical interconnections may be effected quickly and easily through the use of a number of different interconnection methods. For example, one such method involves first positioning a continuous strip of bottom bus material on a work surface, then positioning a plurality of solar cells atop the bottom bus strip so that each solar cell's bottom electrode contacts the bottom bus strip, next positioning a continuous strip of top bus material atop the solar cells so that the top bus strip contacts each solar cell's top electrode, and finally securing the top and bottom bus strips to the top and bottom cell electrodes respectively by soldering or similar means. The foregoing method for electrically interconnecting a plurality of solar cells in parallel is fast and simple and lends itself well to automated assembly techniques.
A number of other methods are known for electrically interconnecting a plurality of solar cells in parallel so as to produce a parallel array of cells. In general, these methods also tend to be fast and simple and to lend themselves well to automated assembly techniques.
Unfortunately, it has proven to be a significantly more complex matter to electrically interconnect a plurality of solar cells in series so as to produce a series array of cells. This is because in a series array, the top electrode of a first cell must be connected not to the top electrode of an adjacent second cell, but rather to the bottom electrode of the adjacent second cell, while the bottom electrode of that first cell must be connected not to the bottom electrode of the adjacent second cell, but rather to the top electrode of an adjacent third cell, or to a load. As a result, the simple and straightforward interconnection methods available to produce a parallel array of solar cells are not applicable to produce a series array of solar cells.
Nevertheless, a number of different methods have been used to electrically interconnect a plurality of solar cells in series so as to produce a series array of cells. One such method involves the use of a plurality of short, separate bus strips to electrically interconnect the cells in the array, wherein each bus strip runs between the top electrode of one cell and the bottom electrode of an adjacent cell. The array is assembled by positioning a first solar cell on a work surface, then positioning a first bus strip on the work surface so that a portion of the first bus strip overlies and contacts the first cell's top electrode, then positioning a second cell on the work surface so that the second cell's bottom electrode overlies and contacts the free end of the first bus strip, then positioning a second bus strip on the work surface so that a portion of the second bus strip overlies and contacts the second cell's top electrode, then positioning a third cell on the work surface so that the third cell's bottom electrode overlies and contacts the free end of the second bus strip, etc. At some point in the process, all of the bus strips are securely attached to the cells' electrodes by soldering or similar means. See, for example, U.S. Pat. No. 4,430,519 (Young).
Another known method for electrically interconnecting solar cells in series also involves the use of a plurality of short, separate bus strips, wherein each bus strip runs between the top electrode of one cell and the bottom electrode of an adjacent cell. According to this method, however, the bus strips are all first securely mounted to a substrate. Then the cells are laid down so that each cell has its bottom electrode overlying and contacting one end of a bus strip. Thereafter, the other ends of the bus strips are made to overlie and contact the top electrodes of adjacent cells. Finally, the solar cells are securely attached to the bus strips by soldering or similar means. See, for example, U.S. Pat. No. 4,019,924 (Kurth).
Unfortunately, neither of the foregoing methods for electrically interconnecting a plurality of solar cells in series is relatively fast and simple, nor does either method lend itself well to automated assembly techniques.
Other methods used for electrically interconnecting a plurality of solar cells in series so as to produce a series array of cells are believed to be relatively slow and complex and not suitable for an automated assembly line.